1. Field of the Invention
The present invention relates to an A/D converter and, more particularly, to a feedback type A/D converter such as a .DELTA..SIGMA. modulator type A/D converter or a .DELTA. modulator type A/D converter.
2. Description of the Related Art
A .DELTA..SIGMA. modulator type A/D converter is known as a feedback type A/D converter. A conventional .DELTA..SIGMA. modulator type A/D converter is designed such that a differential signal based on a difference between an input signal supplied to an input terminal and a feedback signal as an output signal from a local D/A converter is generated by an adder, the differential signal is input to the local A/D converter through a low-pass filter for integration, and an A/D converter output is obtained from an output terminal.
One-bit converters may be used as a local A/D converter and a local D/A converter in principle. If, however, an integrator used as a low-pass filter is a three or more dimensional integrator, and if a converter having a one bit structure is used, an operation of a .DELTA..SIGMA. modulator becomes unstable. For this reason, a converter having a two or more bit structure is used. If a converter having a multiple bit structure is used, the quantization error is decreased, and the signal amplitude inside the integrator ca be reduced.
Assume that in the above-described conventional .DELTA..SIGMA. modulator type A/D converter, the local A/D converter and the local D/A converter are ideally operated. In this case, providing that an input to the input terminal is represented by u(z); an output to the output terminal, by y(z); a quantization error included in the local A/D converter, by e(z); and the transfer function of a low-pass filter 103, by H(z), the output y(z) is given by the following equation: ##EQU1## Therefore, u(z) is obtained as the output y(z). If the low-pass filter serves as an integrator, since 1/H(z) in the above equations represents a high-pass filter characteristic, a low-frequency-band component of noise based on the quantization error e(z) can be reduced. When an input signal is to be sampled by the local A/D converter, if sampling is performed by using a frequency higher than the Nyquist frequency, i.e., over-sampling is performed, so as to remove components other than those in a required band by means of a digital filter (not shown) connected to the output terminal of the A/D converter, the quantization error can be reduced, thus obtaining a desired S/N ratio. This principle is described in detail in Design Methodology for .DELTA..SIGMA. M BHGWATI P. AGRAWAL, KISHANSHENOI IEEE TRANSACTION on Communication, VOL. COM-31, No. 3, MARCH, 1983, pp. 360-370.
In the above-described .DELTA..SIGMA. modulator type A/D converter, an output signal from the local A/D converter is used as an output signal from the .DELTA..SIGMA. modulator. This signal is converted into an analog signal by the local D/A converter. The analog signal is then used as a feedback signal. Therefore, a conversion error in the local D/A converter appears as an output distortion. In order to solve such a problem, the conventional .DELTA..SIGMA. modulator type A/D converter uses a local D/A converter having precision higher than finally required precision thereof. That is, a D/A converter having a one bit structure is used. If, however, a three or more dimensional integrator is used as a low-pass filter, a D/A converter having a two or more bit structure, i.e., a multiple bit structure, is required, and a D/A converter having precision higher than the finally required precision must be used, in order to stabilize an operation of the .DELTA..SIGMA. modulator, reduce the quantization noise, or decrease the signal amplitude inside the integrator.
Assume, for example, that a D/A converter having a four bit structure is to be used. Even in this case, if precision of 16 bits is the finally required precision, a four-bit quantization level must be obtained with the precision of 16 bits or more. A D/A converter having such a multiple bit structure and high precision is difficult to realize. Even if it is possible to manufacture such a D/A converter, various steps such as trimming of a resistor for generating a reference voltage are required in the manufacturing process, resulting in a considerable increase in cost.
As described above, the conventional feedback type A/D converter such as a .DELTA..SIGMA. modulator type A/D converter requires a D/A converter having a multiple bit structure and precision higher than finally required precision as a local D/A converter. Such a multiple bit D/A converter having high conversion precision requires the steps such as trimming, and hence is difficult to realize. In addition, such a converter is expensive.